1. Field of the Invention
The invention relates to an axial-flow turbine.
2. Description of the Related Art
The essential components of the axial-flow turbines of fluid-flow machines are the rotor with the moving blades, the nozzle ring and the cover for the moving blades. Slight discontinuities, which result in a reduction in the efficiency, occur in the flow duct of such axial-flow turbines due to unavoidable production and assembly tolerances.
An axial-flow turbine of an exhaust-gas turbocharger has been disclosed by EP 806 547 A1, this axial-flow turbine being subjected to relatively high temperatures during operation of the internal combustion engine connected to it. High thermal stresses thus occur in the turbine-side components, such as, for example, the gas-inlet casing, the nozzle ring, the cover and the gas-outlet casing. Since each of these components is at a different distance from the internal combustion engine and since, in addition, different materials are used, the component temperatures accordingly differ. The result is different thermal expansions with relative movements between the individual components, which may lead to screw fractures, gas leakages and component cracks. The design and arrangement of the separating locations of gas-inlet casing, gas-outlet casing, nozzle ring and cover are therefore of considerable importance for the operability of the axial-flow turbine and thus of the exhaust-gas turbocharger.
Especially critical with regard to thermal expansions is the nozzle ring, which is usually cast and is arranged between the fixed casing parts and the rotating moving blades of an axial turbine. EP 806 548 A1 discloses a solution for the simple and reliable fastening of the nozzle ring. To this end, the nozzle ring bears with its outer ring against the cover and with its inner ring against the gas-inlet casing. An axial expansion gap is formed between the outer ring and the gas-inlet casing, and a radial expansion gap is formed between the outer ring and the gas-outlet casing.
However, it has been found that, in particular also in the case of discontinuities in the transition region between the outer ring of the nozzle ring and the cover, which are also caused by thermal expansions in addition to the production and assembly tolerances already described above, a corresponding decrease in the efficiency can be expected.
In addition, Dejc & Trojanovskij "Untersuchung und Berechnung axialer Turbinenstufen" [Investigation and design of axial turbine stages], VEB Verlag Technik, Berlin, 1973, page 452 (FIG. 7.32: II) discloses a device for the reduction of the gap losses caused by the radial clearance of the turbine blades. To this end, the moving blades are arranged to be stepped relative to the guide blades combined in the nozzle ring and have a positive overlap, i.e. the inner contour of the cover is arranged radially further to the outside in the region of the moving blades than in the region of the guide blades.
During dismantling, however, such a configuration has the disadvantage that the axial-flow turbine can only be displaced in the opposite direction to the nozzle ring and not in both directions.